I think the OP addresses this in his second paragraph. In the case where registering is open for the public, it is trivial for a would be attacker to find the

I think the OP addresses this in his second paragraph. In the case where registering is open for the public, it is trivial for a would be attacker to find the password requirements. They only need to go through registration to find the password complexity. This means that you are incurring a usability cost with out a security gain. One limitation of showing the requirements at log in I could think of is that it would require all users to have the same password requirements. This is just a thought, but if the password requirements were random throughout the userbase, it would make it more difficult for your users to use the same password across different sites. For example, if you require 25% of your users to have two capital letters, 25% have 2 symbols .. etc, it is less likely they would have gotten the same requirement from a different site.